A packaged semiconductor device and method of its manufacture

Abstract: For manufacturing a packaged semiconductor device, a lead frame (8) with an electrically insulating strip member (70) and a semiconductor chip (4) is placed in a molding unit having upper and lower dies (11a,11b). The upper and lower dies (11a,11b) have recessed areas (a,b) for determining a size of a cavity of the molding unit different from each other, the size of the cavity being measured in a direction perpendicular to a clamping motion direction of the dies (11a,11b). The lead frame (8) is positioned so that a surface of each lead (5) with the insulating strip (70) member applied thereto is contacted with one (11a) of the upper and lower dies having a larger recessed area and a molding line (ML) of the molding unit intersects the insulating strip member (70). The molding unit is closed to clamp the lead frame (8) to depress and thrust into spaces between adjacent leads that part of the strip member (70) which is outside the molding line and to form the cavity of the molding unit. By injecting a molding material into the cavity, a molding package (2) is provided encapsulating the semiconductor chip and a portion of each lead. The packaged semiconductor device (1) has a flanged side surface (S3) with an insulating strip (7) provided between a step (SS) of the flanged side surface and intermediate portions (Li) of the leads (5) which are between first portions (L1) of the leads encapsulated in the molding package (2) and second portions (L2) of the leads protruding from the flanged side surface. An insulating filler (7') is provided in spaces between those parts of the second portions of the leads which outwardly adjoin the intermediate portions of the leads.

Description: BACKGROUND OF THE INVENTION The present invention relates to a packaged semiconductor device and techniques for manufacturing same and in particular to techniques useful when applied to an LSI package having numerous pins or a small pin arrangement pitch such as a QFP (Quad Flat Package). Dam bars are disposed in a lead frame used for fabricating an LSI package. A dam bar is a member disposed in an intermediate portion of each lead of the lead frame so as to couple leads adjacent to each other and has a function to prevent molten resin injected into a molding unit from flowing out to the exterior of the molding unit through spaces between different leads, when a semiconductor chip mounted on the lead frame is molded with resin. It has further a function to act as a supporting member for preventing deformation of the leads. The dam bars are cut away by means of a cutting tool such as a press, etc., after the semiconductor chip has been molded with resin. However, recently, for lead frames used for fabricating LSI packages involving numerous pins, since an increase in the number of pins or leads and a decrease in the pin arrangement pitch are advanced rapidly, it becomes more and more difficult to cut the dam bars by means of a press. Therefore, in lieu of a lead frame in which dam bars and leads are formed in one body from a metal plate, so-called tape dam techniques have been proposed (JP-A-58-28841, published on February 19, 1983) in which a thin insulating tape is applied to a lead frame having no dam bars, and the tape is pressed between different leads in a resin molding operation to use the pressed parts of the insulating tape as dam bars. According to these tape dam techniques, since the tape is made of an insulating material, dam cut after the resin molding is unnecessary and thus it is possible to increase the number of pins or leads and to decrease the pin arrangement pitch. Techniques similar to the tape dam techniques are disclosed also in U.S. Patent no. 5,525,547 issued on June 11, 1996, U.S. Patent No. 5,106,784 issued on April 21, 1992, and in JP-A-5-136310 published on June 1, 1993. JP-A-2-45961 published February 15, 1990 discloses a packaged semiconductor device in which the molding package has a stepped side surface, to protect the leads from stress. SUMMARY OF THE INVENTION The present invention is based on knowledge of the inventors thereof which will be described with respect to Figs. 1 and 2. Therefore, the structures themselves indicated in Figs. 1 and 2 are not known. Fig. 1 shows a state, in which a lead frame 100 provided with tape dams studied by the inventors is mounted on a molding unit. A molding unit includes an upper die 101a and a lower die 101b and the lead frame 100 is positioned on the upper surface of the lower die 101b. The lead frame 100 includes leads 102 and a die pad portion 103. A thin dam tape 104 made of an insulating material such as polyimide resin is bonded to a part of one of surfaces of the leads 102 of the lead frame 100 by means of an adhesive. A semiconductor chip 105 bonded to the upper surface of the die pad portion 103 is connected electrically with leads 102 through bonding wires 106. In order to mold the semiconductor chip 105 with resin, the upper die 101a and the lower die 101b are brought closely into contact with each other and clamped. Then, the dam tape 104 applied to the one surface (upper surface in the example indicated in Fig. 1) of the lead frame 100 is placed between the upper die 101a and the lower die 101b to be crushed so that spaces between different leads 102 are filled with a part thereof to form a dam. Next, molten resin is injected into a cavity of the molding unit to encapsulate the semiconductor chip 105. At this time, since the spaces between adjacent leads 102 are filled with a part of the insulating tape 104 to form a dam, molten resin will not flow out to the exterior of the molding unit through these spaces. Thereafter, the lead frame 100 is taken out from the molding unit and the leads 102 are subjected to solder plating treatment. Further, unnecessary parts of the lead frame 100 are cut away by means of a press and thereafter, an LSI package is completed by bending the leads 102 in a predetermined form. In the molding unit for fabricating an LSI package such as represented by QFP, the upper die 101a and the lower die 101b are symmetric, as indicated in Fig. 1. It has been found that when the lead frame 100, to one of the surfaces of which the dam tape is applied, is mounted on such a molding unit to be clamped, since the lower surface of a boundary portion between the inner lead and outer lead portions of the leads 102 is clamped directly by the lower die while the upper surface of the boundary portion is clamped by the upper die 101a through the dam tape 104, the inner lead portions of the leads 102 are bent in the (downward) direction opposite to the surface to which the dam tape is applied, as indicated in Fig. 2. It has been also found that, in case where the dam tape is applied to the lower surface of the lead frame, the leads are bent upward. As a result, the bonding wires 106 connecting the semiconductor chip 105 with the leads 102 are deformed (the wire twist being twisted), which gives rise to mold deficiency such as wire shorting and inclination of the die pad portion 103 (and of the semiconductor chip 105). In this case, if dam tapes 104 were applied to both surfaces of the lead frames 100, since load would be applied equally both upward and downward to the boundary portions of the leads 102, no bending of the inner lead portions would be produced. However, by this method, since the insulating tapes 104 are inserted by a press or the like into the spaces between different leads in the directions both upward and downward at clamping in the molding unit, for a lead frame 100 having a small arrangement pitch, leads are deformed in the transversal direction to widen spaces therebetween. An object of the present invention is to provide techniques capable of efficiently preventing bending of the inner leads at clamping of the lead frame provided with dam tape with the molding unit. According to one aspect of the present invention, there is provided a method of manufacturing a packaged semiconductor device, as set out in claim 1. According to another aspect of the present invention, there is provided a packaged semiconductor device as set out in claim 4. Owing to the fact that the recessed areas determining the size of the cavity of the molding unit are different for the upper die and the lower die, since positional deviations or displacement takes place between end portions of inner side walls of the upper die and end portions of inner side walls of the lower die, it is possible by the invention to prevent the inner lead portions of the leads (those parts of the leads which are encapsulated in a mold package) from bending in an upward direction or a downward direction. BRIEF DESCRIPTION OF THE DRAWINGS Figs. 1 and 2 are cross-sectional views of a lead frame showing a method of manufacturing a packaged semiconductor device, which the inventors have studied; Fig. 3 is a cross-sectional view of a packaged semiconductor device according to an embodiment of the present invention; Figs. 4 to 6 are perspective views of a lead frame at several stages in a method for manufacturing a packaged semiconductor device according to an embodiment of the present invention; Fig. 7 is a cross-sectional view of a lead frame placed in a molding unit by a method of manufacturing the packaged semiconductor device according to an embodiment of the present invention; Fig. 8 is a cross-sectional view of a lead frame and an insulating strip, when the molding unit shown in Fig. 7 is in a clamped state; Fig. 9 is a cross-sectional view of a packaged semiconductor device obtained by the fabrication method shown in Figs. 4 to 8; Fig. 10 is a cross-sectional view of a packaged semiconductor device obtained by shaping leads of the device shown in Fig. 9; Fig. 11 is a cross-sectional view of a packaged semiconductor device indicating a state where it is under an electrical test; Fig. 12 is a cross-sectional view of a packaged semiconductor device indicating a state where it is being handled; Fig. 13 is a cross-sectional view of a packaged semiconductor device according to another embodiment of the present invention; and Fig. 14 is a cross-sectional view of a packaged semiconductor device according to another embodiment of the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 is a cross-sectional view of a QFP-type packaged semiconductor device encapsulated with a molding resin (hereinbelow called QFP for the sake of simplicity), which is an embodiment of the present invention. A molding package 2, e.g., made of epoxy resin has two main surfaces S1 and S2 opposite to each other and a flanged side surface S3 and a die pad 3, a semiconductor chip 4, e.g., made of a silicon single crystal, and inner lead portions (first portions) L1 of a plurality of slender leads 5 are encapsulated therein. The semiconductor chip 4 is bonded to the die pad 3 by means of an adhesive, e.g., made of epoxy resin. The inner lead portions L1 of the leads 5 are connected electrically with the semiconductor chip 4 through bonding wires 6 made of Au, Cu or Al. The flanged side surface S3 of the molding package 2 has a first side surface portion S31 adjoining the main surface S1, a side surface portion S32 adjoining the main surface S2, and a step portion SS connecting them. An electrically insulating strip 7 is disposed between the upper surface of intermediate portions Li (portions between the inner lead portions L1 and the outer lead portions, i.e. second portions, L2) of the leads 5 and the molding package 2, and an electrically insulating filling material or a filler 7' exposed to the exterior of the package 2 is disposed between those parts of the outer lead portions L2 which adjoin the intermediate portions Li of the leads 5. This strip 7 is made of a thin tape of an insulating material such as polyimide resin and bonded to the upper surface of the leads 5 by means of an adhesive. The filling material or filler 7' is made of the same material as the strip 7. The lower surface of the intermediate portions Li of the leads 5 is not covered by the package 2. As indicated in Fig. 3, in the QFP 1 of the present embodiment, outer sizes of the molding package 2 are different on the upper side and the lower side of the leads 5. More specifically, in this package 2, the outer size on the upper side of the leads 5, to which the insulating strip 7 is applied, is greater than the outer size on the lower side of the leads. That is, the first side surface portion S31 is outward with respect to the second side surface portion S32. The QFP 1 may be manufactured by a method described below. At first, a lead frame 8 as indicated in Fig. 4 is prepared. This lead frame 8 includes a die pad 3, supporting leads 9, each of which is disposed at each corner of the die pad 3 to support it, a plurality of slender leads 5 arranged around the die pad 3, a strip member 70 such as a rectangular frame-shaped dam tape applied to one surface of the leads 5 of the lead frame 8 in a direction approximately perpendicular to the length directions of the leads, and an outer frame 10 supporting the leads 5 and the supporting leads 9. The lead frame 8 is made of a metal such as 42 alloy or Cu and the different members such as the die pad 3, the leads 5, the supporting leads 9, the outer frame 10, etc. are formed in one body by a press or etching. As an example, the lead frame 8 is about 0.1 mm thick, the inner lead portions of the leads 2 are about 0.13 mm wide, and the arrangement pitch of the inner lead portions is about 0.3 mm. On the other hand, the dam tape 70 is about 50 µm thick and about 0.5 - 2.0 mm wide, while the adhesive layer bonding the dam tape 70 to the leads 5 is about 25 µm thick. Subsequently, as indicated in Fig. 5, the semiconductor chip 4 is bonded to the die pad 3 of the lead frame 8 (pellet bonding) and then as indicated in Fig. 6, the leads 5 are connected electrically with the semiconductor chip 4 through bonding wires 6. Next, as indicated in Fig. 7, the lead frame 8 is mounted on a molding unit. This molding unit includes a pair of dies, i.e., an upper die 11a and a lower die 11b, and the lead frame 8 is positioned on the upper surface of the lower die 11b. At this time, the dam tape 70 applied to the upper surface of the leads 5 is in such a positional relation that it intersects a molding line ML of the molding unit. As indicated in the same figure, recessed areas of the upper die 11a and the lower die 11b determining the size of a cavity in this molding unit are different from each other. More specifically, the size (a) of the recessed area of one of the dies of the molding unit, which faces the surface of the lead frame 8, to which the dam tape 70 is applied, namely, the upper die 11a in Fig. 7, is greater than the corresponding size (b) of the lower die 11b. The size of the cavity of the molding unit represents the size thereof measured approximately perpendicularly to the direction of the clamping motion of the dies of the molding unit. Subsequently, as indicated in Fig. 8, the lower surface of the upper die 11a is closely brought into contact with the upper surface of the lower die 11b for a clamping operation. Then, that part of the dam tape 70 bonded to the upper surface of the leads 5 of the lead frame 8 which is outside the molding line ML is sandwiched between the upper die 11a and the leads 5 of the lead frame 8 to be crushed so that it is thrust into the spaces between adjacent leads 5 to form the dam, i.e. the filling material or filler 7' (Fig. 3). On the contrary, that part of the dam tape 70 which is inside the molding line ML remains on the upper surface of the leads 5 as the insulating strip 7. At this clamping, the lower surface of the part of the leads 5 covered by the tape 70 is clamped by the lower die 11b, while the upper surface thereof is clamped by the upper die 11a through the soft tape 70. At this time, since there are positional deviations or displacement ("x" indicated in Fig. 8) between the inner end portion of the side walls of the upper die 11a and the inner end portion of the side walls of the lower die 11b, i.e. between the different recessed areas of the two dies, the inner lead portions L1 of the leads 5 are not bent downward. Under the condition that the above-mentioned numerical values are used, it is verified that no bending of the leads 5 is produced, if these deviations (x) in the size are at least 0.2 mm - 0.3 mm. Then, molding material (molten resin) is injected into the cavity of the molding unit to mold the semiconductor chip 4. At this time, since the spaces between different leads 5 on the outer side of the mold line ML (on the outer side of the intermediate portions Li of the leads 5) are filled with a part of the tape 70 to form the dam, i.e. the filling material (filler) 7' (Fig. 3), molten resin will not flow out to the exterior of the molding unit through these spaces. Thereafter, when the lead frame 8 is taken out from the molding unit, a package 2 having different outer sizes on the upper side and the lower side of the leads 5, i.e. a flange on its side surface, as indicated in Fig. 9, is obtained owing to the differences of the recessed areas of the upper die 11a and the lower die 11b of the molding unit. The second portions (outer lead portions) L2 of the leads 5 protrude from the step portion SS of the side surface S3. On the other hand, the upper surface and the side surfaces of the intermediate portions Li between the first portions (inner lead portions) L1 and the second portions L2 are covered by the insulating strip 7 and the molding material 2, while the lower surface thereof is exposed. Then, after the outer lead portions L2 of the leads 5 have been subjected to solder plating treatment, unnecessary portions of the lead frame 8 are cut away by means of a press or the like. The QFP 1 indicated in Fig. 3 is completed by further bending (shaping) the leads 5 in a predetermined form. In case where the bending direction of the outer lead portions is reversed, the QFP 1 as indicated in Fig. 10 will be obtained. As described above, according to the present embodiment, since it is possible to prevent efficiently bending of the inner leads at the clamping of the lead frame 8, to which the strip member 70 such as the dam tape is applied, by means of the molding unit, highly reliable QFPs 1 can be manufactured with a high yield. Although a lead frame 8, to the upper surface of which a strip member 70 is applied, is used in the present embodiment, in case where a molding is effected with a lead frame to the lower surface of which the strip member 70 is applied, it is possible to prevent the inner lead portions of the leads 5 from bending in the (upward) direction opposite to the surface to which the strip member is applied, by using a molding unit in which the size (a) from one end of the inner side surface of the upper die to the other is smaller than the corresponding size (b) of the lower die. Further the surface of the leads to which the strip member 70 is applied, is not necessarily the surface of the leads to which the semiconductor chip 4 is bonded, but the strip member 70 may be applied to the surface of the leads to which the chip is not bonded, depending on the field of utilization. Further, since the QFP 1 has different outer sizes on the upper half and the lower half sides of the mold package 2, electrical tests can be effected, e.g., by bringing probes 12 of a tester into contact with relevant leads 5 on the step portion SS connecting the side surface portions S31 and S32, as indicated in Fig. 11. In this way, since it is not necessary to bring the probes 12 into contact with readily deformable extremities of the leads 5, it is possible to prevent surely deformation of the leads 5 at electrical tests. Although Fig. 11 shows a QFP after the lead shaping, it is clear that a similar effect can be obtained also in case where electrical tests are effected before the lead shaping. Further, when the QFP 1 is handled by using a jig 13 as indicated in Fig. 12, since it is easy to grasp the narrower (second) side surface portion S32 of the flanged side surface of the mold package 2 by means of the jig 13, handling and transportation of the QFP can be effected more conveniently. In addition, in case where the leads 5 are bent in a J-shape, if they are bent so as to face the inner (second) side surface portion S32 of the flanged side surface of the mold package 2 as indicated in Fig. 13, since a protruding amount of the leads 5 in the transversal direction can be reduced with respect to that required in case where they are bent in the opposite direction, an area required for mounting the QFP 1 can be decreased. Still further, as indicated in Fig. 14, since heat produced by the semiconductor chip 4 can be conducted to a heat radiator 14 through the die pad 3 by mounting the heat radiator 14 on the upper surface of the package mounted on a printed board, it is possible to provide a QFP 1 having a small thermal resistance. Although the invention made by the inventors has been concretely explained in the above on the basis of the several preferred embodiments, the present invention is not restricted thereto, but it is a matter of course that various modifications can be made without departing from the scope thereof. Although, in the above embodiments, the present invention has been explained in case where it is applied to manufacture of QFPs, the present invention can be applied to various sorts of LSI packages mounted on lead frames by the tape dam method.

Claims: A method of manufacturing a packaged semiconductor device comprising the steps of: providing a lead frame (8) without a dam bar, said lead frame having a plurality of leads (5), a die pad (3), and an electrically insulating strip member (70) applied to a first surface of said leads of said lead frame so that the strip member extends in a direction generally transverse to the longitudinal direction of the leads; bonding a semiconductor chip (4) to said die pad of the lead frame; electrically connecting said leads and said semiconductor chip; placing said lead frame with said electrically insulating strip member applied thereto and with said semiconductor chip bonded thereto in a molding unit having an upper die (11a) and a lower die (11b), said upper and lower dies having respective recessed areas (a, b) determining the size of a cavity of said molding unit in the direction substantially perpendicular to the direction of a clamping motion of said dies, said respective areas (a, b) being different from each other, said lead frame with said electrically insulating strip member and said semiconductor chip being positioned such that said first surface of said leads with said electrically insulating strip member applied thereto is contacted by that one (11a) of said upper and lower dies which has the larger recessed area and that a molding line (ML) of said molding unit intersects said electrically insulating strip member (70); and closing said molding unit to clamp said lead frame in such a manner as to depress and thrust into spaces between adjacent leads that part of said electrically insulating strip member which is outside said molding line (ML), and to form said cavity of said molding unit, the part of the insulating strip member which is inside the molding line remaining on the surface of the leads as an insulating strip, and subsequently injecting a molding material into said cavity to provide a molding package (2) encapsulating said semiconductor chip and a first portion (L1) of each of said leads (5), said molding package being stepped at its side surface with a second portion of each said lead protruding from a step (SS) of said stepped side surface. A method according to claim 1, further comprising the step of shaping said second portions of said leads after the molding. A method according to claim 1, wherein a portion (Li) of each of said leads (5) inside said molding line (ML) is supported by a flat surface of the die (11b) having the smaller recessed area, preventing the first portion (L1) of each of said leads from bending, in the step of closing the molding unit to clamp said lead frame. A packaged semiconductor device comprising: a semiconductor chip (3) : a plurality of leads (5) having first and second main surfaces and being electrically connected to said semiconductor chip, each of said leads including a first portion (L1), a second portion (L2) and an intermediate portion (Li) therebetween; a molding package (2) encapsulating said semiconductor chip and said first portion of each of said leads, said package having opposite two main surfaces (S1, S2) and a stepped side surface including a first side surface part (S31) continuing from one (S1) of said main surfaces of said package, a second side surface part (S32) continuing from the other (S2) of said main surfaces of said package and a step (SS) connecting said first and second side surface parts, said first side surface part being outward with respect to said second side surface part to form said step, said second portion (L2) of each of said leads protruding from said side surface of the molding package at said step of said side surface; an electrically insulating strip (7) provided between said step of said stepped side surface and first main surfaces of said intermediate portions (Li) of said leads, second main surfaces of said intermediate portions of said leads being not covered by said molding package; and an electrically insulating filler (7') provided in spaces between those parts of said second portions of said leads which outwardly adjoin said intermediate portions of said leads, said electrically insulating filler being made of the same material as that of said electrically insulating strip. A device according to claim 4, further comprising a heat dissipator (14) provided on one of said main surfaces of said molding package. A device according to claim 4, in which said second portion of each of said leads is bent to be J-shaped opposing said second side surface part (S32) of said stepped side surface. A device according to claim 4, in which said molding package is a quad flat package.

Inventors: HAYASHIDA TETSUYA, KISHIKAWA NORIO, YOSHIDA IKUO, HAYASHIDA, TETSUYA, KISHIKAWA, NORIO, YOSHIDA, IKUO
Applicants: HITACHI LTD, HITACHI, LTD.
Application year: 1994
Main classification: H01L21
IPC codes: H01L23, B29L31, B29C45, H01L21